Also, Microsoft does hold an operating system monopoly. Not a total monopoly, obviously, because I'm typing this on a non-Windows computer, but it is still significant.
While there are some legal definitions of monopoly power, etc. I'm not a big fan of calling something a monopoly that obviously isn't. While small parts of the overall share, Linux and particularly OSX are clearly real alternatives to Windows. The only thing holding them back are a lack of standardization in Linux's case and high prices and high profit margins over a push for market penetration in OSX's case. In other words, they aren't doing what is required to even try and get higher market penetration, so you cannot blame the current market share situation entirely on Microsoft. Even how they got to where they are in the present is based largely on the bad decisions of competing companies during the 1980s and 1990s. Most of Microsoft's market power abuses had little to do with those early companies (e.g. Commodore, Apple and Atari) and more to do with things that came later (e.g. Netscape's complaint about IE being shipped with Windows, dealer contracts that insisted they not include other OSs like Dr. Dos or Linux, etc.)
From Apple to Commodore to Atari, they all had one thing in common. They released their operating system ONLY for their own hardware and that is the primary area Microsoft gained a huge advantage early on by sheer market share. You can argue all day long about anti-competitive behavior, but that only applied to operating systems that ALSO ran on the same Intel/AMD hardware. One cannot complain that Microsoft had ANYTHING to do with the fact that AmigaDos shipped on Amigas and MacOS on Macs and TOS on Atari ST. They didn't stop people from becoming Amiga dealers, for example. My local Amiga dealer also sold PCs (including those made by Commodore).
You
could argue that IBM backing MS-Dos had a LOT to do with its acceptance (and computers in general) in business, however and THAT was more important back then than anything else. After all, Commodore sold more
home computers that anyone else during the 1980s, but being known for gaming and demos doesn't do you much good for getting your office work done at home (as that work started migrating out of the office). Ultimately that fact along with bad business management is what did Commodore in, despite a superior platform to nearly everything out there in the late 80's. Add to the fact that what made the Amiga special was its custom chip set is also what made it hard to upgrade in terms of sound and video (at least for the integrated consumer type Amiga models), which delayed it getting 256 color mode (which PC gaming was heading full steam ahead into by the early '90s) and you had a recipe for disaster. Atari 8-bit was popular in the U.S., but the ST was never very popular over here compared to Europe. It had a niche in early MIDI recording due to the built-in MIDI ports, but that's not enough to differentiate it and in other areas it paled next to the Amiga (but that only split those interested in graphics and sound between them).
The PC's survival was inevitable due to its entrenchment in business (and that may be also the reason Apple is the one of the other three to survive as the Mac DID have significant penetration into some types of businesses that Commodore and Atari largely did not have). Otherwise, MS-Dos would have died in the '80s due to inferior graphics and sound options at the time. The Mac, however, had a shot at being THE system for business in the '90s, but due to Apple's short-sighted decision early on to NOT push it into high market share in favor of high prices and therefore high profits is what killed
any chance of it getting enough market share to surpass MS-Dos despite a superior (graphical) interface. When Windows came out, that was the final nail in the coffin. You cannot compete with high volume cheap computers without something major to differentiate you from the competition.
Unfortunately, that is the very situation Apple now faces again in a new market (the mobile "iOS" device market). Apple wants high profits so they do not push iOS to other hardware manufacturer devices in favor of selling iPhones and iPads at the highest possible profit margins. The problem is Google is NOT going to go that direction and thus the writing is on the wall all over again. The Mac started out strong in the mid '80s and ended up irrelevant by the end of the '90s. Likewise, the iPhone has revolutionized the smart phone industry, but I feel it will also end up irrelevant in the next ten years because Android will be EVERYWHERE and available for nearly every device sold that isn't made by Apple or RIM. Assuming the feature of the OS catches up quickly, the hardware will most certainly far surpass Apple's hardware in short order due multiple manufacturers competing for the most features, etc. and Apple being stuck in longer product cycles since it's the only manufacturer of the iPhone. Prices will most certainly be cheaper on the Android platform due to competition since they will run the same OS.
In short, it is the SAME thing all over again and history is going to repeat itself with Apple again. This time people won't be able to say that "if only Steve Jobs were at Apple, Microsoft never would have won" because IMO it never mattered whether he was there or not in the 1990s. Maybe the operating system would have done better sooner, but the marketing strategy would have been exactly the same and the prices would have been just as high and so Apple never would gained the foothold on market share that Microsoft has. In fact, even today when they had a chance to regain a lot of that lost market share while Vista was very unpopular, they chose to raise prices instead of lowering them and to not compete directly with small micro-towers that have been the hallmark of home Windows based PCs for nearly two decades! Yes, this gave them high profits again, but profitability by itself can be fleeting over time whereas market share ensure long term survivability.
If my prediction is true and the iPhone/iOS market eventually loses most of its early share to Google/Android based devices for the same reasons that Apple lost its ~20% share to <3% share (high prices and lack of hardware competition), Apple could find itself back to 1998 in a very short period of time. Any gains the Mac got due to Vista will start evaporating as they spend all their efforts trying to keep iOS devices from falling behind multiple companies all working to out-do each others' hardware for Android OS. So with little to no personal computing share as Windows steams past a floundering Apple that is putting all its efforts into gadgets instead of computers and diminished iOS share due to one company not being able to compete with multiple companies doing the same thing for less, Apple will have to either innovate into new areas (maybe Apple branded waffle makers?

) or evaporate into obscurity once again.
I'd prefer that NOT happen as I do want choices other than just Windows. Stagnation is likely to occur with only one operating system being out there. I can imagine Google pushing its own flavor of Unix or Linux in the future, however. They seem to be heading in that direction and unlike Apple, the wouldn't have a vested interest in selling hardware, thus creating a conflict of interests for the best possible operating system). In fact, if someone would just force Linux to use ONE standard set of libraries, package managers, etc. and could get a large market share in place, it could be competitive, but short of someone like Google that has a lot of clout, I don't see that happening since there's just too many distributions and competing standards. Linus Tovalds probably could have mandated a set of core standards, but he clearly has no interest in such a thing. Many Linux people are anti-commercial enterprise anyway and couldn't care less if something like Photoshop or various games are EVER available for Linux. For the rest of us, the lack of commercial software is a platform killer. But I fear Apple is just too greedy to ever choose lower prices or license the OS out to other hardware vendors in order to get higher market share so it'll either remain a profitable niche market or eventually face obscurity once again.